This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
In the field of flow measurement, it is often necessary to condition the flow upstream of a flow metering device in order that the flow meter will register flow with a minimal error. Bends, valves, filters and other forms of pipeline component distort the flow velocity profile and by changing the flow direction introduce non-axial velocity components or ‘swirl’ in the flow stream. It is well known that the calibration or flow coefficient of certain types of flow meter is affected by distortions of the profile and/or by the presence of swirl. Flow conditioners have been employed for many years to partially rectify distorted and swirling flows upstream of flow meters. The various devices deployed to date differ in design with resulting differences in performance in terms of their ability to rectify flow versus the permanent pressure loss that they impose. Most conditioners have a single specified geometry or a constrained set of design parameters and cannot easily be adapted to suit the requirements of a particular situation. The invention described here aims to overcome these and other limitations of existing conditioners.
Flow conditioners have been used for many years to attempt with the aim of rectifying incoming flow conditions and improving flow meter accuracy. By far the most common type of flow conditioner has been a ‘flow straightener’ of either the vane type or in the form of a tube bundle assembly. Flow straighteners essentially divide the flow into a number of passages that are long and straight in parallel with the axis of the pipe. The aim is that any rotational component of velocity is reduced or eliminated when the flow exits the conditioner.
The tube bundle is the most commonly employed form of flow straightener, having been standardized to some degree, and is essentially an assembly of tubes, typically between 7 and 55 in total, arranged either in a hexagonal or circular geometry, as illustrated in FIGS. 1a and 1b. A tube bundle using 19 tubes of equal size arranged in a circular geometry is included in the International Standard for differential pressure flow meters, ISO5167. Tube bundles are typically made to be between two and three pipe diameters in length, with the result that the tubes may be 20 to 30 tube diameters long, though studies have shown that in terms of limiting swirl, a much shorter length of bundle can still be effective.
A recognized deficiency of the tube bundle design of flow conditioner is that while it is effective at removing swirl, the emerging axial velocity profile does not tend to be fully developed, that is it generally tends to be flatter than the profile that would be found downstream of a long straight length of pipe at the Reynolds number of interest. In order to try to overcome this limitation, Stuart developed a tube bundle flow conditioner where the tube diameters used within the bundle were varied in order to produce a velocity profile shape closer to the desired fully developed profile. A disadvantage of this conditioner design in terms of manufacturing, which also applies to most tube bundle designs, is that when pipe diameter is varied, the required tube diameters may not be readily available in standard sizes of tubing. The main advantage of tube bundles is that they have relatively low permanent pressure loss, having a loss coefficient for fully turbulent flow in the range of 0.65 to 1.2.
A further disadvantage of the tube bundle is its variable design and quality. If not constructed to the ISO standard, the potential variations in number and size of tubes are almost endless, making it difficult to predict performance or relate experience from one design of tube bundle to another. Furthermore, variable manufacturing quality means that the tube alignment may vary, and in some cases, for example if the bundle becomes twisted during manufacture, the bundle can produce a swirling flow.
The need to shape the axial velocity profile as well as remove swirl was probably first addressed properly in the design of the Zanker flow conditioner. The Zanker conditioner comprises a thin plate with holes designed to produce a graded resistance to flow combined with a vane type straightener attached to the downstream side of the plate. In terms of the flow profile produced and level of swirl reduction achieved by this conditioner, it is recognized as being very effective. However, it is somewhat difficult to manufacture and has a high pressure loss coefficient of greater than 5.
More commonly used today are thick-plate type conditioners. In these designs a graded resistance to flow is achieved by means of making circular passages in a fairly thick plate. By varying the number, spacing and/or size of the circular passages, the desired graded resistance is achieved. Examples of this type of conditioner include those by Laws (most common in the Nova/CPA 50E variant), Spearman, and Gallagher, in addition to the thick plate version of the Zanker conditioner, where the thicker plate negates the requirement for the downstream vane-type straightener. Common thick-plate conditioners are illustrated in FIGS. 2a-2d. 
These thick-plate conditioners with circular passages are considered the current state-of-the-art but still have certain deficiencies. Pressure loss coefficients are typically in the range of 2 to 5, greater than that available with a tube bundle. Attempts to produce plates of higher porosity and hence lower pressure loss have generally resulted in a reduction in flow conditioning performance.
Optimization of the design of a thick-plate conditioner with circular passages is complicated by particular issues associated with the chosen circular hole geometry. An irregular numbers of holes and the circular shape of the passages result in a complex ‘water-shed’ between adjacent rings of holes, and hence makes the calculation of the effective porosity difficult as the water-shed defines the blockage area associated with each hole. Optimization is further complicated in cases where the circular passage size is varied, as for a given thickness of plate as this results in variation of both the porosity and the ratio of the length of the passage to its hydraulic diameter. As a consequence, the steps that should be taken to optimize a conditioner with circular passages are not obvious, as when changes are made the shape of the water shed varies as well as the porosity and the hydraulic diameter.
A particular advantage of the thick-plate conditioner is that the manufacture and geometric scaling to different sizes of pipe can be achieved very easily, which overcomes the manufacturing and quality limitations associated with the tube bundle type of conditioner.
As mentioned previously, the effectiveness of thick-plate conditioners has been found to diminish when the porosity is increased too much, with the result that most thick-plate conditioners in use today have porosity in the region of 50%. When porosity has been increased, the investigators have not tended to increase plate thickness to compensate for the reduction in l/d, which may partly explain the diminished performance. This has led some designers to add straightening vanes to the conditioner or to employ two stages of conditioning, the first being a straightening vane and the second a graded thick-plate conditioner.
Some types of flow meter are more affected by the condition of the incoming flow field than others. In the case of multi-path ultrasonic flow meters, it is often the case that if swirl is removed effectively then the meter will be able to perform with high accuracy in a variety of different installation conditions. Therefore, it is common for tube bundles to be used with ultrasonic meters, owing to their lower pressure loss characteristics. However, this does not offset three of the disadvantages of tube bundles: first, that they alter the axial velocity profile in an adverse way; secondly the fact that they are generally manufactured to be between 2 and 3 diameters long, and thirdly the manufacturing issues mentioned above that can result in poor quality conditioners. Therefore, it is one purpose of the invention described here to be able to produce a low pressure loss flow conditioner for use with ultrasonic and other types of flow meters. In addition to having a low permanent pressure loss, the conditioner should be easy to manufacture in a reproducible way and it should be possible to vary the design parameters in order to obtain a desirable shape of axial velocity profile.